A large number of patents and publications found in the literature, which relate generally to the manufacture of LiPF.sub.6, are only concerned with the synthesis of phosphorus pentafluoride (PF.sub.5), without in any way expanding upon the method used in order to react this product with lithium fluoride (LiF). Thus, Patent JP-41 75 216 essentially provides a method for the preparation of PF.sub.5 without metals, without SO.sub.4.sup.2- ions and without heavy products; Patent JP-52 79 003 is targeted at producing PF.sub.5 which is free of the impurity POF.sub.3 ; Patent JP-65 6 413 is targeted at producing PF.sub.5 without PF.sub.3 Cl.sub.2 ; German Patent Application DE-A-196 14 503 also provides a method for accessing a purer PF.sub.5.
The other patents and publications, which expand in greater detail upon a process for the synthesis of LiPF.sub.6 itself, generally provide methods which are complex, of little use industrially and liable to introduce impurities. Thus, for example, U.S. Pat. No. 3,594,402 provides a method for the preparation and/or purification of LiPF.sub.6 which involves the intermediate stage of a complex between acetonitrile and LiPF.sub.6 ; U.S. Pat. No. 3,607,020 provides for carrying out the reaction between solid LiF and gaseous PF.sub.5 in an organic solvent in which LiF is insoluble, PF.sub.5 is soluble and LiPF.sub.6 is "reasonably" soluble; Japanese Patent JP-60 251 109 provides for directly reacting solid PCl.sub.5 with a lithium salt in solution in HF; Japanese Patent JP-64 72 901 provides a method which makes it possible to render solid LiF porous and then to react it with gaseous PF.sub.5.
All these methods are very complex to industrialize because they make use of processes with multiple stages and/or involving solid/gas or solid/liquid reactions and/or requiring the use of intermediate organic solvents.
The quality requirements demanded by the market for LiPF.sub.6 mean that the process implemented must be as simple as possible, that is to say minimize the number of unit operations, and that it must avoid operations in which solids are directly handled and the production line is correspondingly opened: the particular aim of this is to avoid the entry, into the process, of air and moisture capable of generating hydrolysable oxygen-containing impurities incompatible with the purity specifications desired for the final LiPF.sub.6.
In its research into a method which would make it possible to obtain LiPF.sub.6 in a simple way, Applicant considered, for this synthesis, bringing gaseous PF.sub.5 into contact, and causing it to react, with LiF predissolved in HF. However, this operation of bringing into contact poses several practical problems:
first of all, the simple sparging of PF.sub.5 into an LiF+HF solution inevitably results in blockages, a phenomenon not mentioned in the literature: this is explained by the speed of the LiPF.sub.6 formation reaction, on the one hand, and by the limited solubilities of LiF and of LiPF.sub.6 in HF, on the other hand; PA1 subsequently, the reaction is rapid and exothermic: it is thus advisable to take measures to control the rise in temperature of the mixture, in order to avoid either excessive PA1 evaporation of the HF, which would then immediately cause recrystallization of the lithium salts and consequent blockages, or overheating detrimental to the safety of the plant and to the stability of the LiPF.sub.6 formed.
The most economical methods for accessing PF.sub.5 naturally include those which start from the cheapest starting materials, which are PCl.sub.3 or PCl.sub.5. The PF.sub.5 thus formed is then accompanied by HCl. Having studied the HF/PF.sub.5 /HCl equilibria, Applicant has shown that it is impossible to separate HCl in any simple way from PF.sub.5 ; it is therefore necessary, in the stage of synthesis of LiPF.sub.6, to employ a method which is compatible with the presence of this HCl, in other words which can start from a crude PF.sub.5 resulting from a reaction employing PCl.sub.3 or PCl.sub.5.